Dental implants which are currently in use are in general made of a metal, e.g. titanium, or a ceramic, e.g. a zirconia based ceramic.
In contrast to metal implants, which are dark and therefore mismatch with the color of natural teeth, ceramic materials have the advantage that their color can be closely matched to the natural tooth color. Efforts have thus been made to provide dental implants, of which at least the parts that are visible after insertion are made of a ceramic material.
Despite their favourable properties with regard to the color, the use of ceramic materials for dental implants is in many cases limited by their fatigue stability, which is generally rather low.
A ceramic material having a high mechanical strength is disclosed in U.S. Pat. No. 6,165,925, which relates to an yttrium oxide-stabilized zirconium oxide in predominantly tetragonal form (yttria-stabilized tetragonal zirconia; Y-TZP) for the production of a sintered semi-finished article.
Despite its favourable mechanical properties, in particular its high strength, toughness and wear resistance, yttria-stabilized tetragonal zirconia (Y-TZP) has however a propensity to low-temperature degradation (LTD) in the presence of moisture, as for example described by Chevalier et al., J. Am. Ceram. Soc., 92 (9), 1901-1920 (2009).
Low-temperature degradation is a kinetic phenomenon in which polycrystalline tetragonal zirconia transforms to monoclinic zirconia over a rather narrow but important temperature range, typically from room temperature to about 400° C.
The degradation progresses from the surface of the material to its interior and is accompanied by micro- and macrocracking, thus resulting in reduced fracture strength of the material.
This problem, which is also referred to as “low hydrothermal stability”, is particularly relevant for the use of zirconia for dental implants, as thereby the material is exposed to a humid and warm environment and needs to fulfil relatively strict safety requirements over a long period.
In addition, dental implants are often subjected to a subtractive treatment in order to improve its osteointegrative properties. In this regard, EP-A-1 982 670, for example, relates to a process for providing a topography to the surface of a dental implant made of a ceramic material, wherein at least a part of the surface is etched with an etching solution comprising hydrofluoric acid. Etching of the ceramic material has however been found to often go along with a further deterioration of its hydrothermal stability.
For improving its hydrothermal stability, it has been suggested to dope yttria-stabilized zirconia with a suitable amount of ceria. In this regard, it is referred to Huang et al. Journal of the European Ceramic Society 25 (2005), pp. 3109-3115 and to Settu et al., Journal of the European Ceramic Society 16 (1996), pp. 1309 to 1318, both referring to yttria-ceria-stabilized zirconia.
Yttria-ceria-stabilized zirconia has however the disadvantage that it is darker in colour than yttria-stabilized zirconia. This is particularly disadvantageous if the material is used for a dental implant, which preferably has a light colour matching with the natural tooth colour. Further, yttria-ceria-stabilized zirconia has the disadvantage that it is not suitable for being subjected to hot-isostatic pressing (HIP) after sintering. Thus, the strength obtainable for yttria-ceria-stabilized zirconia is lower compared to yttria-stabilized zirconia.
Alternatively, a homogenous dispersion of Al2O3 grains into a tetragonal yttria-stabilized zirconia matrix has been reported to increase the hydrothermal stability of the tetragonal phase, as for example stated in the above mentioned article of Huang et al. However, also the doping with alumina has a negative impact on the translucency of yttria-stabilized zirconia.